A logarithmic A/D converter has been used to perform A/D conversion on a detection signal that has a wide dynamic range. This is because a logarithmic A/D converter is capable of providing more sufficient resolution for infinitesimal input than a linear A/D converter, and performing A/D conversion on a higher inputted voltage without becoming saturated. Japanese Laid-Open Patent Application H01-158878 discloses an example of such conventional logarithmic A/D converter. FIG. 6 is a circuit diagram showing this conventional logarithmic A/D converter. As shown in the figure, a reference voltage “Vref” is applied to a reference voltage terminal 44 of the logarithmic AID converter, and divided by a plurality of resistors 47 having different resistance so that divided voltages form a progression corresponding to a logarithmic function. Voltage divided by each of the resistors 47 is compared by comparators 42 with an input voltage “Vin” inputted via an analog input terminal 45. In accordance with a result of this comparison, each of the comparators 42 outputs a logic signal set at either high or low. A decoder 43 converts these logic signals into a digital signal “Dout” represented in binary code, and the converted digital signal “Dout” is outputted from a digital output terminal 46. In this way, the above conventional A/D converter divides the reference voltage “Vref” by using resistors 47 to produce divided voltages that form the logarithmic function progression, and outputs the digital signal “Dout”.